For ensuring the continuity of voice service between a circuit switched (CS for short) domain and an internet protocol (IP for short) multimedia subsystem (IP Multimedia Core Network Subsystem, IMS for short) domain, traditional solution is as shown in FIG. 1, that is the solution of voice call continuity (VCC for short).
FIG. 1 is a principle diagram of VCC in the existing IMS. In FIG. 1, dual mode user equipment (UE for short) is successfully registered in both the CS domain and the IMS network. When initiating voice service in the CS domain or the IMS domain network, the UE is anchored to the service centralization and continuity application server (SCC AS for short) in IMS network, at this time the UE has two known SCC AS numbers, a session transfer URI (STI for short) and a session transfer number (STN for short), wherein the STI is used to indicate switching the user session from the CS domain to the IMS domain or a packet switched (PS for short) domain on a service layer; and the STN is used to indicate switching the user session from the IMS domain to the CS domain on a service layer. When UE prepares to switch an established session from CS domain to IMS domain or from one packet switched domain to another packet switched domain, the US initiates a session request to the SCC AS in the IMS network by using the STI as an indication. After receiving the session request, the SCC AS switches the access of established session from the CS to from the PS or from one PS to another PS. Similarly, when the UE initiates a voice service in the IMS network and prepares to be switched from the IMS domain to the CS domain, the UE uses the STN as a called number in the CS domain and initiates a call for the SCC AS. After the SCC AS receives the session request, the voice service is switched from an IP access to the CS domain, therefore the entire switch is completed.
Above description is for the realization principle of traditional VCC, wherein the dual mode UE must simultaneously support the working mode of dual radio, that is, the dual mode UE can simultaneously perform voice call in the IMS domain and the CS domain.
With the flat development of network architecture and the emergence of various high-speed wideband movable access modes, the mobile communication network is impelled to provide next generation architecture concept of LTE/SAE, including that the evolved radio communication system is enabled to provide higher transmission rate, shorter transmission delay, and lower cost and at the same time support the mobility between the access systems in 3rd generation partnership project (3GPP for short), the mobility between 3GPP access system and non-3GPP access system and the like. At the same time, LTE/SAE will also be used as one access manner of IMS domain to support the existing VCC function. Due to the large energy consumption of mobile phone battery under dual mode and strong mobility management ability of LTE/SAE per se and the like, a single radio voice call continuity (SRVCC for short) is proposed. The principle architecture for realizing SRVCC solution is provided hereinafter. FIG. 2 is a related SRVCC principle diagram based on SAE network architecture. As shown in FIG. 2, corresponding network elements of SAE comprises:
an evolved universal terrestrial radio access network (E-UTRAN for short): evolved radio access network, that is next generation radio access network, which can provide higher uplink and downlink speed, lower transmission delay and more reliable radio transmission;
a mobility management entity (MME): control plane functional entity, which is in charge of managing and storing UE context (such as UE/user identifier, mobile management status, user safety parameters and the like), allocates temporary identifier for the user, and is responsible for user acknowledge when the UE resides on the above-mentioned tracking area or network;
a system architecture evolution gateway (SAEGW for short): user plane functional entity, for user plane data route process, which is divided into a serving SAEGW (SGW for short) and a packet data network gateway (PDNGW for short), wherein SGW is mobile anchor points between the SAE system and the traditional 3GPP system and between E-UTRANs; PDN GW is used for terminating the downlink data of UE in idle state; and when the downlink data transmitted to the UE is arrived, PDN GW is used for initiating paging, managing and storing UE context (such as IP bearer service parameters and network interior routing information).
The existing 2G/3G mobile network comprises:
a radio access network comprising a 2G GSM EDGE radio access network (GERAN for short) and a 3G universal terrestrial radio access network (UTRAN for short); core network comprising a circuit switched domain network element (MSC) and a packet switched domain network element serving GPRS support node (SGSN for short), wherein MSC provides circuit voice service, circuit data service and short message service, and SGSN is connected with SAE GW in LTE/SAE and is used for providing packet switched domain service;
home subscriber server (HSS for short) managing the data of circuit switched (CS) domain, packet switched (PS) domain and multimedia subsystem (IMS) domain.
The IMS network comprises:
core network elements of core network element call session control function (CSCF for short), media gateway control function (MGCF for short) and the like, and an SCC AS for realizing VCC function.
The UE can access the IMS domain via LTE/SAE. The IMS domain can provide the IMS service for the UE. The UE also can access the IMS network via the PS network (that is, GERAN/UTRAN and SGSN parts as shown in FIG. 2) of 2G and 3G. The IMS network can provide the CS service, such as voice service, for UE.
The existing network in the prior art is described in the above. In order to realize the continuity of single radio voice service, the existing MSC has been enhanced. The enhanced MSC is connected with the core network element MSC of the CS domain of 2G/3G, core network element of the PS domain of 2G/3G and the MME of the LTE/SAE network, and controls the handover between the LTE/SAE network and the 2G/3G network. The enhanced MSC also can be connected with the SCC AS via the MGCF in IMS domain network to ensure that all call can be anchored to the SCC AS in the IMS domain and ensure realization of the basic functions of the VCC, wherein the interface with the MSC adopts an E interface, and the E interface herein uses a mobile application part (MAP) signaling. The enhanced MSC and the MSC, performing the handover using inter-office handover mode of the CS domain, has the E interface used therebetween, and if there is only the enhanced MSC, the message interaction therebetween will be realized interiorly.
Telecommunication network must provide emergency voice call service. When user dials emergency service number or uses the default emergency service number stored in terminal device, or initiates the emergency service by using emergency service identifier, the emergency service will be connected by the network to the local emergency communication acceptance center (such as police, fire alarm, medical aid, rescue and the like).
In prior art, the operation that the emergency service visits the domain is realized in the way that: the emergency service accesses, by a proxy-call session control function (P-CSCF), to an emergency-call session control function (E-CSCF), and further accesses, by the E-CSCF, to a local emergency service center (Presentation-Service Access Point, PSAP for short), and thereby the emergency service is realized.
FIG. 1 is a principle diagram of the VCC under the existing IMS, which is used for realizing the continuity of single radio emergency service. As shown in FIG. 1, the CSCF is the E-CSCF, and the SCC AS is an application server for realizing the continuity of emergency service under the E-CSCF. Such server is located in visit domain, and is variable for each user. The SCC AS is located in home domain, and is fixed for each user.
The enhanced MSC accesses the E-CSCF in the visited IMS domain; the E-CSCF accesses the visited SCC AS; and the PDN GW/S-GW accesses the visited IMS domain.
FIG. 2A is a flow chart of prior art realizing to switch the access from LTE/SAE network to the access from the CS domain based on the SRVCC principle diagram in FIG. 2. As shown in FIG. 2A, a session has been established in the IMS between single UE and UE-B, and is anchored to the visited SCC AS to perform handover from the IMS domain to the CS domain. The prior technical procedure mainly comprises the following steps (S201-S218):
Step S201, after receiving the E-UTRAN measurement request of the LTE/SAE radio network element, the UE sends a measurement report, and the measurement report carries the information of adjacent cell;
Step S202, the E-UTRAN initiates a relocation request message to the MME according to the information of the adjacent cell in the measurement report;
Step S203, the MME sends a forward relocation request message to the enhanced MSC according to the information of the relocation request message; the forward relocation request message carries the domain transfer identifier (STN) of the user and the integrated service digital network (ISDN) number of the user; STN and ISDN numbers are configured according to the user information and the MME, and are obtained via the HSS;
Step S204, the enhanced MSC sends a MAP preparation handover request message to the MSC;
Step S205, the MSC sends a handover requirement message to GERAN/UTRAN, and the GERAN/UTRAN replies a handover requirement acknowledge message to the MSC;
Step S206, the MSC replies a MAP preparation handover acknowledge message to the enhanced MSC, wherein the message carries a handover number (HO#) which is allocated by the MSC;
Step S207, the enhanced MSC sends an initial address message of ISDN user part (ISUP) signaling to MSC according to the handover number, and MSC returns the address complete message of ISUP to the enhanced MSC;
Step S208, the enhanced MSC sends an initial address message of ISUP to the SCC AS via the MGCF and the CSCF in the IMS domain, the message carrying STN and ISDN numbers, wherein, the ISDN number and STN indicates that the SCC AS switches the session of the corresponding user from the IMS domain to the CS domain on service layer, and SCC AS sends the reply message to the enhanced MSC; or the enhanced MSC directly sends an SIP session request message to the SCC AS, and the SIP session request message carries STN, or the ISDN number of the user is switched to a telecommunication universal resource identifier (tel URI), the SCC AS replies the SIP successful message (such as the successful message of the SIP can be represented as 200 OK) to the enhanced MSC;
The enhanced MSC sends the SIP session request message to the SCC AS directly or via the ISUP initial address message in the above-mentioned steps, representing to initiate voice service continuity handover request to the SCC AS, and switch the voice service continuity handover request from the IMS domain to the CS domain on the service layer;
Step S209, the enhanced MSC replies the forward relocation response message to the MME;
Step S210, the MME replies the relocation response message to the E-UTRAN;
Step S211, the E-UTRAN replies the relocation command message to the UE;
Step S212, the US sends handover access message to the target GERAN/UTRAN;
Step S213, after detecting the handover, the GERAN/UTRAN sends a message of having detected the handover to the MSC;
Step S214, the MSC sends a process access signaling message of the MAP to the enhanced MSC, and notifies the MSC that user terminal performs the access from the CS domain;
Step S215, the UE sends a handover completion message to the GERAN/UTRAN;
Step S216, the GERAN/UTRAN sends the handover completion message to the MSC;
Step S217, the MSC sends MAP_SEND_END_SIGNAL message (MAP sending end signal) to the enhanced MSC, and the message notifies the enhanced MSC that the user terminal has been switched to the CS domain;
Step S218, the MSC replies the ISUP response message (corresponding to the ISUP initial address message in step S207) to the enhanced MSC;
The specific handover comprises the following two conditions: one condition is sending the handover request to the MSC via the enhanced MSC, and the other condition is switching to the enhanced MSC without switching to other MSC. In case of the second condition (hereafter referred to as integrated-configuration), steps S204, S206, S207, S214, S217 and S218 will be realized in the enhanced MSC.
According to the above-mentioned steps, the access of the corresponding IMS session from the LTE/SAE network is successfully switched to from the CS domain. The handover comprises two handover procedures: one handover procedure is the above-mentioned step S208 in which the session is switched from the IMS domain to the CS domain on the service layer, and the other handover procedure is the steps, except step S208, in which the access of the session is switched from the LTE/SAE network to from the CS network.
From the above description, it can be seen that the STN in step S208 is obtained from the HSS by the MME in step S203. Since the emergency service is completed in the visited domain, in case that the step S208 adopts the STN obtained from the HSS, the SCC AS in the home domain will fails to associate the session due to non-existence of the emergency service information of the UE, thereby which makes the handover of the entire service failed, and can not ensure the continuity of the emergency service in the single radio channel.